This invention relates to a method for producing crack-free activated carbon structures. It also relates to a method of reducing the the drying time of activated carbon green bodies without the formation of cracks, fissures and other deformations.
Certain metal and non-metal structures are produced from very fine metal, ceramic, glass and carbon powders and a high level of binders and water. Typically, due to the large surface area of the fine powders, the solid matrix requires the addition of a large amount of water sometimes more than 150 weight % based on the weight of powders in the matrix. As a result of the high water content of the matrix, structures formed from the matrix require very long drying times. In addition, structures formed from the matrix tend to develop cracks and fissures during the drying operation due to differential shrinkage. Consequently, the dimensional stability of such structures has remained largely unacceptable, resulting in a large number of such structures being rejected in the drying operation. This problem is particularly severe when forming complex structures such as thin-walled, honeycomb structures formed by extrusion.
Several methods have been suggested to improve the dimensional stability of formed structures. For example, in a composition comprising mainly inorganic powders and 10-30% water, it has been suggested to dry the article by first cooling the article to below 0.degree. C. using either a cooling medium such as liquid nitrogen, acetone, dry ice or by use of freon gas freezer thereby completely freezing the water contained in the molded article, and subsequently removing the water through evaporation in the solid state by placing the completely frozen article in a vacuum of about 10.sup.-3 mmHg.
It has also been suggested to add substantial amounts of other components to the matrix to improve the dimensional stability of the formed articles. For example, it has been suggested to add 1 to 30 parts by weight of a graphite powder to 100 parts by weight of ceramic powder to form a ceramic article. The graphite is intended to reduce the amount of water needed to form the batch and thereby reduce the drying shrinkage and cracking. Others have suggested the addition of thermoplastic or thermosetting resins, and polyhydric alcohol. The above methods have the major disadvantage of adding substantially to the manufacturing cost of such articles due to the added steps of removing the resins or graphite by combustion at high temperatures and for long periods of time.
It is also well known in the art that total drying time can be reduced by raising the drying temperature and lowering the relative humidity of the drying medium. However, when this technique is applied to activated carbon bodies, particularly, large complex shaped bodies such as honeycomb structures, the bodies tend to crack during the drying operation.
Nothing in the background art has suggested a method of producing self-supporting, green activated carbon structures free of cracks and other deformations. It is therefore the object of the present invention to provide a method of producing green activated carbon structures from methyl cellulose-containing batches by which such structures become self-supporting and defect-free in a very short period of time.